1. Field of the Invention
The present invention relates to an apparatus, a method, and a program product for detecting a digital watermark, in which the watermark value is detected from a motion image in which a topological invariant is embedded as the watermark value.
2. Description of the Related Art
Conventionally, there is well known a digital watermark technique. In the digital watermark, pieces of information such as information on an author of copyright to data, user identification information, rights information, utilization condition, necessary confidential information in use, and copy control information are embedded in digital copyrighted work data such as sound, music, video, and an image while the pieces of information are hardly perceived, and the pieces of information embedded in the digital copyrighted work data are detected and utilized if needed. The digital watermark is utilized for the purpose of copyright protection or secondary use of the digital copyrighted work.
It is thought that the digital watermark is associated with a topological invariant by regarding geometric distortion as homomorphic mapping. The inventors already proposed a topological invariant digital watermarking technique in order to realize robustness to geometric distortion (for example, Japanese Patent No. 3431593). In the topological invariant digital watermark technique, an invariable topological invariant (for example, homotopy class) is embedded as the digital watermark under the geometric distortion.
Generally, security measures against a process performed in a time axis direction (hereinafter such processes are referred to as “time axis direction attack”) such as frame rate conversion and frame thin-out can be realized by embedding the same watermark value in plural frames. However, in this case, it is difficult to distinguish the embedded watermark values whether the watermark values are embedded as the same watermark value or different watermark value. Therefore, it is necessary to detect switching of the watermark values.
In a technique in which a homotopy class is used as the topological invariant, one digital watermark value is realized by embedding and extracting three kinds of components of (X,Y,Z). In a technique of embedding one component in one frame or a technique of utilizing an inter-frame difference to embed one component, when the digital watermark suffers from the time axis direction attack, there is generated a problem that distinction cannot be made among the X, Y, and Z components, and thereby the watermark value cannot correctly be detected.